I run a campaign (Pathfinder mostly, with several house rules) in which I had an idea to incorporate a shadow/red dragon hybrid. It goes well with the story and the entire concept of the setting, but I was unable to find any official rules about mixing dragons of different colors and/or types.

So, if no such rules exist, what would be the easiest way of "mixing" these? And in case they do, where do I find them (preferably with an example or two).